Recording from Multiple Webcams Synchronously while LSL Streaming¶
Info documents¶
location Repository: https://github.com/sarkadava/multiple_webcam_recording_for3Dtracking
location Jupyter notebook: https://github.com/sarkadava/multiple_webcam_recording_for3Dtracking/blob/main/webcam_scripts.ipynb
Requirements¶
Please install the packages in requirements.txt
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citation for this module: Kadavá, S., Snelders, J., Pouw, W. (2024). Recording from Multiple Webcams Synchronously while LSL Streaming [the day you viewed the site]. Retrieved from: https://envisionbox.org/multiple_webcam_record.html
Introduction¶
In some of the modules on envisionBox we perform 3D tracking on multiple cameras that are recording synchronously. Before we started using those types of methods, we found that it is non-trivial to do actual recordings from multiple cameras in a synchronous way. Therefore we share a script here that allows to record from three webcams while also streaming information about the framenumbers to an LSL stream.
Through trial and error, we found that ffmpegcv was the most stable solution for recording 3 webcams simultaneously in a synchronous way. A good way to test whether your webcams are synchronous is holding a stopwatch in front of all cameras, recording the videos, and compare for each frame if all videos show the same time.
LSL stream, what is it good for?¶
Often you want to combine audio and other signals with video. LabStreamingLayer is a very robust solution for this. LSL operates by collecting signal streams. In this script we create one such stream, whereby we stream the frame number f(t) at some time t. In LSL recording (using LSL labrecorder) you can write the stream to a file, and then LSL will store the framenumber alongside a common timestamp. You can then later align your frame number with the common time stamp, thereby ensuring a) that even when frames are dropped or collected with different intervals you can give the actual time of the frame recording, and b) you can align your other signals with very high precision to the frame (which may for example be the basis for your kinematic measurements). For example, we might also stream a accelerometer signal on a different PC on the network, and this stream will also be collected with a LSL recorder and given a common timestamp. Since the acceleration signal and the framenumbers are timestamped with a common clock (note different systems generally have different clocks) they can be synced and aligned.
We are here assuming that you are already working with LSL. At some later moment we might do a LSL tutorial if needed. If you just want to use the script for recording that is fine too, you could either leave the script as is, or you can comment out parts that refer to LSL (and just write the videos to a disc).
Acknowledgements¶
We want to thank Pascal de Water/) at the Donders Institute for Brain, Cognition and Behaviour for first helping us to a demo script that does streaming and webcam recording. The current script is a heavily adapted version (making use of ffmpegcv instaed of opencv2) of that original script.